Polyarylene sulfides (hereinafter, may be abbreviated as “PAS”) as represented by polyphenylene sulfides (hereinafter, may be abbreviated as “PPS”) are engineering plastics having excellent heat resistance, chemical resistance, flame retardant properties, mechanical strength, electrical properties, dimensional stability, and the like. PAS can be molded into various molded products, films, sheets, fibers, and the like by a general melt processing method such as extrusion molding, injection molding, compression molding, and the like, and therefore are widely used in a wide range of technical fields such as electrical apparatuses, electronic apparatuses, automotive apparatuses, packaging materials, and the like.
A well-known representative example of a PAS manufacturing method includes a method of separating and recovering PAS through washing and drying from a reaction liquid containing PAS obtained by polymerization reacting a dihalo aromatic compound (herein after, may be abbreviated as “DHA”) such as paradichlorobenzene (hereinafter, may be abbreviated as “pDCB”) or the like, and a sulfur compound such as an alkali metal sulfide, alkali metal hydrosulfide, or the like as a sulfur source under heating conditions, in an organic amide solvent such as N-methyl-2-pyrrolidone (hereinafter, may be abbreviated as “NMP”) or the like.
The polymerization reaction is a desalting polycondensation reaction, and for example, an alkali metal halide (such as NaCl) or other byproduct alkali metal salt, dimer, trimer, or other low polymer, impurity (such as volatile substance and substance with a high boiling point), or the like is generated in addition to the PAS as a reaction product, and after the reaction, unreacted raw material remains. Therefore, the organic amide solvent, byproduct alkali metal salt, low polymer, impurity, or the like is present between particles or inside the particles of the PAS after the polymerization reaction, or in the reaction liquid. Therefore, PAS separated from the reaction liquid containing PAS is recovered after washing thoroughly to remove the organic amide solvent, byproduct alkali metal salt, low polymer, impurity, and the like, and thus the quality of the PAS as a product is improved and maintained.
The organic amide solvent removed by the washing is preferably recovered and re-used from the perspective of resource saving, cost reduction, and the like. Examples of methods of recovering an organic amide solvent from washing wastewater containing the organic amide solvent using a solvent containing water as a washing liquid in the aforementioned washing in order to remove the organic amide solvent include: methods of directly distilling the washing wastewater; methods of extracting using a specific organic solvent such as branched alkyl alcohol or the like (for example, Patent Documents 1 to 3); and the like.